By C. Kohn, Waterford, WI
 An animal’s body must have consistency – a body’s
physiological variables must be kept within a narrow
range
 For example, you must always keep blood sugar, dissolved
oxygen, the pH, and the salinity of the body’s tissues at a
consistent level
 When these variables cannot be controlled, a disease will
occur
 E.g. Diabetes is a disease in which the body cannot control
the levels of sugar (glucose) in the blood
 There are two systems responsible for maintaining
homeostasis (homeostasis is maintaining a constant internal
environment)
 Nervous system – brain, spinal cord, nerves
 Endocrine system – endocrine glands
(pancreas, thyroid, adrenal glands,
ovaries/testicles) and hormones
 The nervous system is responsible
for quick, split second responses,
such as reflexes and reactions.
Source: users.rcn.com
Source: biology.clc.uc.edu
 The endocrine system is responsible
for very long-term responses; for
example, puberty and sexual maturation
occur over a period of years in humans
and other animals.
 The endocrine system coordinates bodily responses with
the nervous system, but each works in different ways
 The endocrine system functions by releasing chemicals
into the blood to change, tweak, or reverse the metabolic
activities of a specific group of cells
 It works relatively slowly and usually the results are long
lasting or even permanent
 The nervous system uses electrical signals that travel at
lighting fast speeds to alter the activities of muscle,
glands, and other structures.
 The results are immediate but short-lived
 A gland is a type of organ that secretes a substance into
another part of the body
 E.g. your salivary glands secrete saliva into your mouth
 There are two types of glands
 Exocrine – releases a substance into an empty space in
the body (e.g. the salivary glands releasing saliva into the
mouth, or sweat glands onto the surface of the skin)
 Endocrine – releases substances called hormones
directly into the bloodstream
– e.g. your pancreas secretes
insulin directly into your
blood to control
blood sugar levels
Source: commonsensehealth.com
 The endocrine glands
include
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Pineal
Hypothalamus
Pituitary
Thyroid
Parathyroid
Adrenal
Pancreas
Ovaries or Testes
 Note that some parts of
the brain are considered
part of the endocrine system
and not the nervous system!
 Hormones are created in one of three ways
 Amine-hormones are derived from the amino acid tyrosine
 For example, epinephrine (adrenalin) is created in this way
 Peptide or Protein-based Hormones – these are derived
from proteins created by the body
 E.g. growth hormone is created from a protein
 Steroid hormones – these are created from cholesterol
 E.g. testosterone and progesterone are created in this way
During the estrus cycle, hormones control signs of
heat, the release of the egg from the ovary,
maintain pregnancy, and regulate almost all
aspects of reproduction.

they are: chemical messengers secreted by tissue (an
endocrine gland) and carried by blood to different
tissue (a target tissue) with a direct specific effect.
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Endocrine gland – not a specific organ, just an organ that
secretes the hormone in the specific case
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~
(just like Air Force One is not a specific plane but the name given to
any plane carrying the president)
Endocrine gland and target tissue can even be the same thing
Tissue only responds to a hormone if it has a
receptor for that particular hormone
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works sort of like a lock and key
a hormone is the key, but a key can only work if it has a
lock to work on
tissue can have (and usually does have) multiple
receptors for multiple organs
Summary: an endocrine gland releases a hormone
into the blood, which
travels throughout the
body and affects tissue
with a receptor for that
particular hormone with
a specific targeted effect.
GnRH – gonadotropin releasing hormone
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Regulates the secretion of FSH
Injections of GnRH cause developing eggs in follicles to
be ovulated (released) and stimulates the growth of
immature eggs in new follicles ~
 FSH – Follicle Stimulating Hormone
 Stimulates growth of eggs in follicles on the ovary
 LH – leutenizing hormone
 Promotes ovulation (release of eggs from the follicle) ~
Estrogen – jack of all trades
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Causes behavioral changes in the cow
Causes LH surge just prior to release of follicle
Coordinates acceptance of bull and release of follicle
Necessary to ensure that sperm and egg meet
Stimulates muscular contractions that move egg into
oviduct and contractions in vagina, cervix, and uterus
to move sperm towards oviduct ~
Progesterone
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Prepares uterus for pregnancy
Prevents development of new follicles
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Prevents recurrence of estrous cycle during pregnancy ~
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 Prostaglandins (PGF2α)
 In non-pregnant cows, uterus secretes prostaglandins
 Cause the corpus luteum to regress and stop producing
progesterone
 Follicles are no longer inhibited and can grow and develop
 In animals with a mature corpus luteum, a heat typically
occurs 2-7 days after an injection of prostaglandins ~
 Day 20 and 21 (of previous cycle) – Corpus luteum
decays; final maturation of a follicle for the next
cycle begins
 Day 1 and part of Day 2 (30 hours total): estrus (heat
period), the point of sexual receptivity.
 The only time that a cow will allow herself to be mounted.
 Egg and follicle reach final maturation. ~
 Day 2-5 [Starting 10-14 hours after last sign of
estrus]: follicle is expelled from the ovary into the
oviduct (a.k.a. ovulation).
 Wall of ruptured follicle that remains on the surface of the
ovary will become the corpus luteum.
 Days 6-16: corpus luteum is large and fully
functional.
 Some growth of follicles which will not fully mature until
current corpus luteum regresses, at which time a dominant
follicle will emerge and eventually ovulate in the next
cycle. ~
 Days 16-20: Corpus luteum will start to regress. The
uterus will begin to try to detect the presence of an
embryo.
 if no embryo is detected, the uterus will send a signal to
the corpus luteum telling it to regress so that a new cycle
can begin.
 If egg has been fertilized, the cow is pregnant and the
corpus luteum will continue to produce hormones
(progesterone) that inhibit follicular development.
 Day 20 – 21 – Corpus Luteum decays if there is no
fertilized egg;
 final maturation of a follicle for the next cycle begins. ~
 Knowledge of the physiology and endocrinology of the
reproductive tract has resulted in the creation of
numerous pharmaceuticals that aid in regulating,
adjusting, or terminating the estrus cycle
 This can be of great advantage to the producer.
 All treatments must be thoroughly researched and
extensively tested to determine whether or not they are
safe for administration to animals and must show that
they cause no harmful effects
in humans who consume products
from those animals
 All of these products available for
sale must be FDA approved.
 If a producer misses a cow in heat, or if the
heats are not easily detectable, profits can
quickly be lost if the cow is not bred in a
timely fashion.
 Research at UW-Madison led to the creation of Ovsynch,
a hormone therapy treatment to synchronize ovulation
and allow for predictable, timed inseminations.
 In short, we can use this treatment
to make a cow go into heat at a
specific time that is known in
advance
 This can greatly increase the
chances of impregnating the cow
Source: animal.ufl.edu
 Ovsynch uses two hormones
 PGF2α and GnRH
 Ovsynch occurs in three stages
Source: genex.crinet.com
 Day 0 - Stage 1: GnRH injection to create a new follicle
 Day 7 - Stage 2: PGF2α injection to end the currnet estrus
cycle and regress the corpus luteum
 Day 9 – Stage 3: Second GnRH
injection to cause the new follicle
to ovulate and release the egg
 All cows will ovulate 24-32 hours
after the second GnRH injection
 Day 10 – Insemination
Source: animal.ufl.edu
 CIDRs (Controlled Internal Drug Release) are an
intravaginal progesterone insert used in the beef cattle,
dairy cattle, goat and sheep industries.
 The progesterone is released at a controlled rate into the
bloodstream after insertion.
 In all species, CIDRs are used for the synchronization of
estrus.
 This can be highly beneficial
in large herds because with the
synchronization of estrus, groups
of cows and heifers can be bred at
the same time in a narrow window.
Source: northsbreedingsolutions.com.au
 CIDRs are coated with progesterone.
 In vivo, progestrone functions to ‘progress’ or sustain the
pregnancy.
 Progesterone will prevent the animal from going into
heat.
 When the CIDR is removed at
the end of a treatment period,
the progesterone production also
immediately stops
 This enables ovulation to occur
Source: ladyofag.wordpress.com
 All animals on the CIDR treatment will go into heat at
the same time, reducing labor costs
 The animal will respond to the progesterone from the
CIDR in the exact same way it will respond to
progesterone from its
own body
 The two hormones are
physiologically
indistinguishable
Source: drugs.com
 Lutalyse is a veterinary pharmaceutical brand name
 Lutalyse is a PGF2α therapy treatment
 If an animal is known to be open (i.e. her ovulated egg
was not inseminated), a shot of Lutalyse will end the
cycle and start a new one.
 Lutalyse/ PGF2α will cause the death of
the corpus luteum, resulting in reduced
progesterone production.
 Pregnant women SHOULD NOT
administer shots of Lutalyse; it can
cause their bodies to terminate the
pregnancy!
 Ovsynch – ends current cycle, and re-starts a new
follicular wave at a predictable time
 CIDR – a vaginal insert used to delay estrus to a certain
time so that all animals in a herd will be able to be bred
at the same time
 Lutalyse – a PGF2α treatment used to end a cycle in an
open, unbred cow